28 Prof. Forbes's Experiments on the Electricity of Tourmaline 



charged with vitreous or resinous electricity ; and the repulsions 

 occasioned by the presence of the body under experiment, are 

 measured with sufficient exactness by the deviation of the needle 

 reckoned on the divided circle of paper ik. I may add, that the 

 graduated circle H at the top of the glass tube is employed to 

 measure any required torsion of the silk fibre produced by causing 

 the cork F to revolve. By means of this simple apparatus, I 

 obtained results of greater accuracy than my objects generally 

 required # . 



Upon presenting to the electrometer, which is quite an insu- 

 lator, an energetic crystal of tourmaline (which should not be too 

 thin), heated to a considerable temperature, the gilt disk being 

 charged with the same electricity as is acquired in cooling by 

 the pole of the crystal presented, the following results appear : — 

 At first the tourmaline exerts no influence whatever. But after 

 its temperature has begun to fall, the repulsive power is gradual- 

 ly developed, and the gilt disk slowly recedes as the increasing 

 force appears ; the recession becomes very minute, and at length 

 reaches a maximum at which the needle remains for a time sta- 

 tionary. Soon, however, a diminution of repulsion takes place, 

 the disk reapproaches its original point of rest, and, if left long 

 enough, will return to the zero point precisely opposite the crys- 

 tal, which then as at first produces no action whatever. It is 

 unnecessary to point out how completely this verifies M. Bec- 

 querel's views, and demonstrates that, as soon as the tempera- 

 ture completely ceases to change, not the minutest vestige of 

 electricity remains, though the insulation throughout should 

 be maintained as completely as possible. This I generally ac- 



* It is surprising that Coulomb's instrument has not been more employed in 

 this inquiry. Becquerel seems only to have used it once, and Dr Brewster had 

 recourse to the laborious and unsatisfactory method of causing pyro-electric crystals 

 to lift fragments of the Arundo Phragmites, which can give no comoarative re- 

 sults. 



